Scanning probe microscopies (SPM) encompass a suite of techniques capable of probing surfaces with high spatial resolution.1-2 Atomic force microscopy (AFM) revolves around sensing mechanical forces between the tip and the sample and affords great flexibility in sample composition and working environment.4 AFM measurements on samples ranging from solid-state materials to soft biological tissues have been reported in environments ranging from vacuums to aqueous solutions.5-6 
Hybrid techniques incorporating AFM have been developed.2-3,7-8 For example, in chemical force microscopy (CFM), tips modified with specific functional groups have been used to probe adhesion or frictional forces between the tip and surface of interest.9-10 More specific interactions have also been probed with tips functionalized with antibodies or other specific recognition sites.10-11 As another example, electrochemical measurements have been integrated with AFM to develop scanning electrochemical microscopy (SECM-AFM).8, 12-13 This enables for simultaneous measurement of topography and electrochemical properties at the mesoscale. Variations of this approach have been applied to imaging redox-labeled nanoparticles and, in the life sciences, for studying enzyme activity and cellular oxidation events.14-15 As another example, metal coated AFM tips have been used to develop tip-enhanced Raman spectroscopy (TERS).16 Most work in this area, however, has focused on combining fluorescence sensitivity with the high-resolution force mapping. This combination is particularly informative in biological samples where correlations between fluorescently labeled species and surface topography can yield new structural insights. The most successful implementation is near-field scanning optical microscopy (NSOM) which uses specially fabricated fiber optic probes to deliver light to the nanometric dimension.17 This approach provides high-resolution fluorescence and topography information with single molecule detection limits.18 
Refractive index probes a fundamental parameter of a material. Mapping the refractive index of a material is useful given recent developments in photonics and energy applications.20-22 Refractive index is also used to monitor binding events at surfaces for applications in sensing and screemng.23-25 Unlike fluorescence measurements, refractive index sensing does not require an intrinsic fluorophore or external tag and is not limited by photobleaching. Refractive index sensitivity with AFM has been implemented using both surface plasmon resonance and ellipsometric approaches.26-29 